1. Field of the Invention
This application is generally directed to the field of rotary screw compressors and more particularly to means for providing positive guidance for a slide valve or a slide stop and a slide valve assembly which are utilized to govern the capacity and the compression ratio of compressors. More specifically, the present invention is directed to providing specially configured bushings having outer arcuate guide surfaces for engaging arcuate upper surfaces of a slide valve or slide stop and slide valve assembly to ensure a close non-interfering tolerance between the slide stop, slide valve and intermeshing rotors of the screw compressor.
2. Brief Discussion of the Related Art
Rotary screw compressors of the type which are utilized for compressing a refrigerant gas or other fluid conventionally include a pair of rotors having intermeshing lands and grooves helically disposed about the periphery thereof. A working fluid enters the compressors from an inlet port and enters the grooves between the rotors. The rotation of the rotors forms somewhat chevron-shaped compression chambers in which the working fluid is received and which chambers diminish in volume as the chambers move toward an outlet in the compressors"" outlet casings. The working fluid is discharged when the crest of the rotor lands defining the leading edge of a compression chamber pass an edge of the discharge ports in the discharge casings of the compressors.
To provide a controlled variation in volumetric capacity simultaneously with controlling compression ratio, it has been known to utilize a combined valve, otherwise known as a slide valve and slide stop which are axially aligned with one another within a recess or bore which communicates along its full length with the bores of the compressor in which the rotors are mounted. The end of the slide valve is moveable relative to the discharge port and its position is used to govern the compression ratio of the compressor. Further, by spacing the slide stop and slide valve relative to one another, it is possible to control the amount of working fluid being compressed and thus the overall capacity of the compressor. When the slide stop and slide valve are spaced relative to one another a space is created therebetween which communicates the area of the intermeshing rotors with the inlet side of the compressor thus preventing any compression in the spaces between the lands and grooves between the slide stop and the slide valve. Maximum capacity is achieved when the slide stop and slide valve are abutted in an end-end relationship with respect to one another thereby preventing any of the working fluid from exiting the compression chambers created between the lands and grooves along the length of the rotors after entering the compressor inlet. Therefore, the slide valve configurations of rotary screw compressors allow an infinitely variable capacity to be maintained which simultaneously results in very efficient compressor loading.
Various means have been employed to guide the slide valve and slide stop to prevent contact or interference with the rotors. An essential in conventional compressors is to retain the slide valve in a position for insuring very tight clearances with respect to the rotors. One such method for accomplishing this guidance task is by machining a guide surface into the inlet and discharge casings for the compressor housing which have the same radius as the top concave sides of the slide valve and slide stop. In this respect, it should be noted that the upper surface of the slide valve assembly is defined having two concave surfaces which meet at a central crest or apex which is designed to extend closely to, but intermediate, the intermeshed rotors. The prior art guide surface is machined by circle interpolation relative to bearing bores provided in each of the inlet and discharge casings of a compressor. The centers are located utilizing separate dowel pins between portions of the central housing of the compressor and the inlet and outlet end casings. Due to the number of possible tolerances which must be considered, there is generally no way of precisely measuring the guide surface position when machining the guide surfaces. Thus, with conventional compressors, some hand fitting has necessarily been required during compressor assembly to provide for accurate tolerances between the surfaces of the slide valve and slide stop and the rotors.
The tolerances involved with current compressors include the following:
for the discharge casing; the tolerances associated with the guide surface radius at the discharge casing, the guide surface to dowel pin center, the bearing bore to dowel pin center, and the bearing bore to guide surface relationship.
For the rotor housing; the dowel pin center to rotor bore, the rotor bore to slide valve bore and the slide valve bore tolerance.
For the slide valve; the slide valve outside diameter tolerance as it relates to the rotors, the slide valve center to bore center tolerance and the tolerances associated with the bores of the rotors.
In view of the foregoing, there remains a need to provide for a closer and more accurate guiding of the slide valves and slide stops of rotary screw compressors without requiring additional manual modification of the components during assembly.
The present invention is directed to a method and apparatus for providing accurate guidance and close tolerance for and between a slide valve or an assembly, including slide valve and slide stop, and the working rotors of a rotary screw compressor. The invention provides for a first pair of bushings to be mounted within bores in which the support bearings for the rotors are located. Each of the first bushings includes an inner annular sleeve which is machined to provide a close fit within the bearing bore within the inlet casing of the compressor. Each of the first bushings further includes an outer semi-annular flange which extends radially outwardly with respect to the central axis of the bushing relative to the annular sleeve such that an arcuate convex surface portion of each bushing provides a guide surface for engaging an arcuate concave portion of one half of the upper surface of a slide valve or slide stop. The guidance surface portion of each bushing is machined so as to have a radius substantially equal to the radius defining the arcuate concave upper surface portion associated with the slide valve and slide stop.
Each of the first bushings also includes a flat extending along the outer flange so that when the bushings are mounted within adjacent bearing bores, the flats oppose each other in closely spaced relationship and thus the flanges of the bushings do not interfere with one another.
The invention further contemplates the use of second bushings adapted to be installed in bearing bores associated with the discharge casing of the compressor. Each of the second bushings includes a central annular sleeve adapted to be closely fitted by machining within the bearing bores in the discharge casing and further include outer semi-annular flanges which extend outwardly relative to an axis with respect to the inner sleeve thereof. The flanges also include arcuate convex guide surfaces which are utilized to engage and guide an upper arcuate concave surface portion of the slide valve with the radius and curvature being machined with respect to the guide surfaces of each bushing to be substantially the same as the radius defining the curvature of the upper concave surfaces of the slide valve. As with the first bushings, flats are provided in each of the flanges so that when the bushings are installed within the bushing bores in the discharge casing, there is no interference between the two flanges. In order to prevent leakage, in some embodiments, a recess or groove is formed in one or both of the opposing flats of the flanges with an appropriate sealing material being placed within the groove.
The second bushings also are profiled to provide fluid passageways in the event the flanges extend in communication with any portion of the discharge outlet through the discharge casing of the compressor. The profiled openings are provided through the outer flanges.
It is a primary object of the present invention to provide a method and bushings for use with rotary screw compressors which will ensure an accurate guidance for a slide valve or assembly of a slide valve and slide stop associated therewith to thereby provide optimum tolerances between the slide valves and slide stops and the intermeshing rotors of the compressors without any interference between these components.
It is also an object of the present invention to guide components for accurately positioning slide valves or assemblies of slide valves and slide stops relative to the intermeshing rotors of rotary screw compressors wherein guide surfaces are created by accurately machining bushings which are installed in bearing bores of compressors so that guide surfaces thereof match the configuration and tolerances associated with the rotors of the screw compressors and with the arcuate surfaces associated with the slide valves and slide stops.
It is yet a further object of the present invention to provide guidance bushings for use with rotary screw compressors wherein the bushings not only provide means for providing accurate guidance for the slide stops and slide valves associated therewith, but which also include annular outwardly extending flanges which may also be utilized to accurately align sections of compressor housings as the housings are assembled.
It is a further object of the present invention to provide guidance bushings for use with rotary screw compressors for ensuring accurate tolerances and positioning of slide valves and slide stops relative to the intermeshing rotors of the compressors wherein the bushings can be utilized and installed interchangeably as wear occurs.
It is another object of the present invention to provide guide bushings for use in rotary screw compressors for accurately guiding slide valves and slide valve and slide stop assemblies such that there is a substantial reduction in manually altering of component tolerances during compressor assembly.